Food packages that are heated to prepare their contents for consumption are known. For example, some foods, such as popcorn, are packaged in bags that are placed within a microwave oven for heating. As the popcorn kernels are heated and pop, the bag expands. Sometimes the contents in these packages may be overheated or insufficiently heated because the consumer does not know when the package has reached an appropriate temperature for consumption of the contents. Displaying a message to indicate to a consumer when the package and its contents have reached a predetermined temperature is a desirable goal.
In U.S. Pat. No. 5,083,815 to Scrymgeour et al. a popcorn bag is disclosed that reveals indicia for a number in a lottery type game in response to the bag. The number may be revealed in one of two ways. First, a user can scratch an opaque ink covering the number so the number can be observed. Second, the opaque ink is formulated to become transparent when heated to a predetermined temperature number. Thus, heating the bag enables the ink to reveal the underlying number printed on the bag. While the second method of revealing the number is based on changes in thermal characteristics of an object, it requires the bag to be printed with the number indicia in one type of ink and then be covered with the ink that changes properties to reveal the number when heated. Simplifying the bag manufacturing process to enable the bag or other object to indicate the package and contents are ready at a predetermined temperature remains a goal in the packaging industry.